Fleming’s semicircular indentation

I got pretty excited the first time I saw the Horsehead Nebula. Here are my notes from the early morning of October 13, 1991, which was also the first night I had my then-new 50-cm f/5 Obsession Dobsonian under a dark and transparent sky.

! Horsehead — . . . I carefully pinpointed its exact location with Uranometria and at 53x centred the field in the eyepiece. Then with the 16-mm at 182x and the O III filter I looked — nothing. Rats. Let’s try the UHC — I looked — wait a minute, wait a minute — a little more averted vision, then after all these 23 years I saw it with my own eyes . . . It was quite a bit larger (and fainter) than I expected (which is what most people say, I hear). It was more like a darker notch taken out of the sky rather than a silhouette against a bright nebula. Although that may sound rather contradictory, that was my impression. After looking at it in Chuck Dethloff’s 24-inch and an h-beta filter (which showed it very clearly) he loaned me the filter (which was a 2-inch) and I triedx